Electron discharge tube



March 1946- J. H. o. HARRIES ELECTRON DISCHARGE TUBE Filed Sept. 4, 1941 4 Sheets-Sheet 1 March 19, 1946. J, O HARRlEs 2,396,949

ELECTRON DISCHARGE TUBE Filed Sept. 4, 1941 4 Sheets-Sheet 2 March 19, 1946. J. H. o. HARRIES ELECTRON DISCHARGE TUBE Filed Sept. 4, 1941 4 Sheets-Sheet 5 A TTO/PIVEVS March 1946' J. H. o. HARRIES ELECTRON DISCHARGE TUBE Filed Sept. 4, 1941 4 SheetsSheet 4 INVENTOR A TTOR NEYS snr orric 2,396,949 ELECTRON DISCHARGE TUBE John Henry Owen Harries, Clapham Park, London 8. W. 4, England Application September 4, 1941, Serial No. 409,585 In Great Britain September 5, 1940 Claims.

This invention relates to electrode arrangements for forming and controlling electron beams and can well be applied to electron discharge tubes according to British Patent No. 521,199,

199 mentioned above, and in the specification of British Patent No. 521,246, a method isdescribed of placing a shielding grid between the cathode and the positive or accelerating grid with its wires in alignment with those of the positive grid and maintained at zero or negative potential. The purpose of the shielding grid is to prevent excessive heat loss in the accelerating grid due to electron bombardment. The wires of this grid shield the positive grid wires from the electron stream and reduce the interception of current to a very small amount. This provides a very substantial advance on the ordinary form of positive grid, but both this arrangement and the use of a positive grid alone are, it has now been found, subject to a further disadvantage. The grid meshes running in parallel lines across the cathode form an axially asymmetric assembly which is a bad feature in a beam-forming element.

Thus, electrons drawn from the cathode into the beam by these grids are found, when they have passed through the beam-producing means normally employed, to take widely difierent paths resulting in an indefinite beam having an uneven cross-section and a non-uniform density of current, and therefore not properly focussed. This non-uniformity results in the effective size of the beam being very much larger than is de-- sirable. This difficulty appears to be due to differing velocities of electrons depending on Whether they are drawn through the grid mesh near to or far from the wires of the grid; in other words, what corresponds to severe chromatic aberration is produced.

It appears that this defect is inherent in any system of grid wires in front of the cathode. Apparently it is not possible to avoid the use of a number of grid wires because otherwise, an insuflicientpositive or accelerating field is set up in front of the cathode and insufiicient current is drawn from the cathode.

The present invention aims at arranging the mesh in the grid construction so that the effects of the mesh upon the focus are made useful instead of undesirable. and generally to produce in a deflection valve the maximum output current change for unit input deflection force.

It has now been found as a result of experimental work, that ii the beam of electrons which has been produced by the co-operation of parallel grid wires with a cathode is brought to a focus by a further converging field which is preferably produced by electrodes not having a mesh configuration, then the beam of electrons is spread in cross-section in the direction at rightangles to the direction of the grid wires. This spread is accompanied by a corresponding reduction in the dimension in the direction of the grid wires and a good focus may be produced provided that the mechanical alignment of the electrodes is arranged to be very good and provided that the focus is not disturbed by imperfections occurring in that part of the electrode structure followin the grid wires. Unless these provisions are met. the effect in accordance with the present invention will not be produced. In summary, therefore, provided these considerations are taken into account a ribbon-shaped beam is produced of which the dimension of the cross-section parallel to the grid wires is much smaller than any other dimension.

According to the present invention, therefore. an electrode assembly for forming an electron beam, including a cathode and a positive grid comprising a number of parallel grid wires mounted closely in front of the cathode, and having in front of this positive grid an electron lens system and a beam-deflecting system, is arranged so that the beam-deflecting electrodes produce the deflection in a direction parallel to the direc tion of the parallel grid wires of the positive grid. The electron beam, for the reasons mentioned above, has an approximately ribbon-shaped crosssection after leaving the beam-forming electrodes, and the greatest dimension of its rectangular cross-section is at right-angles to the direction of deflection. It follows that the beam is deflected in the direction in which it is narrowest, and it will be appreciated, therefore, that the current change produced by its movement over an output target or targets is greater than would be the case if the deflection took place in the direction at right-angles.

The arrangement according to the present invention is very effective in producing an accurately-formed beam in comparison with that described in the specification of British Patent No. 521,199, largely because a far greater degree of axial symmetry is obtained. Thus, although intermediate electrodes may be at different potentials from the potential in the immediately surrounding. space, and although space charges may cause the space potential to drop, the beam is not distorted. Therefore,.the beam formation is maintained more eiiectively, and for this reason a beam of ribbon or rectangular section is obtained by means of the arrangements of the electrodes, and not by attempting to give the cathode a particular shape such as a rectangular sec= tion. Thus, a substantial advantage is obtained in regard to the capacity to handle very large currents, since a ribbon section beam is obtained while maintaining the axial symmetry of the electrodes except for the use of the parallel grid wires, This departure from axial symmetry is actually made use of to produce the ribbon section beam so that the unavoidable aberrations due to the unavoidable use of grid wires, are actually usefully employed.

In order that the invention may be clearly understood and readily carried into eiifect, some examples Of constructions in accordance therewith will now be more fully described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a central longitudinal section of a construction of electron discharge tube provided with an electrode assembly in accordance with the present invention;

Figure 2 is a so-called exploded view in perspective showing each oi the electrodes in detail but moved further apart than they are in the tube;

Figure 3 is a longitud nal section taken at rightangles to Figure l of the central portion of the tube;

Figure 4 is a central section to an enlarged scale showing the cathode and the immediately adjacent two grids:

Fi ure 5 is an end elevation on the same scale as Figure 4, showing the re-entrant cathode:

Figure 6 is a central section showing a somewhat modified construction of grid:

Figure 7 is a cross-section of the tube taken on the line VIIVII in Figure 1: and

Figure 8 is a circuit diagram showing the tube connected as a frequency multiplier in a manner suitable for testing purposes.

The sequence of electrodes can be seen best in Figures 1 and 2 and includes a spiral cathode C. the ne ative grid G mounted cl se to the cathode C. and the positive grid G havin t e same functions a described in the spec fication of Bri ish Patent No. 521.199. Next. there is t e deflection elect ode of cyl ndrical sha e formed in tw halves D D to receive the alternatin defle ting potentials. The next e ectrode is the high poten tial electrode L and then the su pressor electrode S. The next electrode is the sub-an de SA with the rectangular aperture or slit s, and finally the anode or target T.

The cathode C. seen best in Figures 2 and 5, is a re-entrant fiat spiral wound in accordance with my copending application Serial No. 409.586, filed September 4, 1941. with its terminals at the outer end of a diameter so as to be substantially non-inductive. It is carried on two lead-in conductors c which are sealed into the bottom seal S of the tube. as shown in Figure 1 and to maintain rigidly the spacing and parallelism of the conductors c, they are fused to glass spacing beads b, b. while they are held in a j A mica disc m is sl pped on to the inner end of the conductors c before the cathode C is welded to them. As best seen in Figure 5, the cathode is a fiat re-entrant spiral wound from tungsten wire, in the particutends backwards around the cathode C for the 1 purpose of confining the electron emission from the cathode.

The positive grid Ci is of similar construction to the grid G and the distance between the faces of the wires of these two grids is 0.4 mm. The

wires g of the positive grid G are the same sizeand spaced apart the same distance, and the aperture diameter is the same as for the grid (3. The details of construction are best seen in Figure 4, which shows that each of these grids consists of a plate-like electrode with an upstanding ring 7' of nickel welded to it. Also, asshown, although this construction does not form part of the present invention, the wires g. g are secured to the ring r at one end by welding, as shown at I, but at the other end, as shown at 2, the wires are free to slide in order to avoid distortion or buckling due to expansion. The wires of each of these two grids are covered over by a thin annular metal mask 9 g around their edges. The wires themselves are of tun sten, having a diameter of 0.4 mm.- 1 In Figure 6. a modified construction, which may be used for both grids G G, is shown. The grid G as illustrated, is shown with an integral part of the plate g pressed out instead of the ring r as shown in Fig. 4. The grid wires 9 are fixed at one end to the punched-out ring g and at theother end are free to slide, as in Figure 4, while the metal mask 9 is placed around the edge 01' the Wires 0 After assembly, both of the forms of grid shown in Figures 4 and 6 are pressed between die of suitable shape to'flatten them, and are then annealed in the usual manner in a non-oxidising gas to remove strains in the metal.

The two halves D ID of the deflection cylinder forming the deflecting and focussing electrode are substantially half-cylinders in shape. although, as can be seen in Figures 1, 2 and 3, the electrode has two cylindrical parts, that of smaller diameter being directed towards the cathode C. The two parts D D are supported on the lead-in conductor d d2 sealed into the side walls of the glass envelope E, and indeed the connection to all the electrodes so far described, except the cathode, are taken out at the side of the envelope. As shown in detail in Figure '1, the lead-in conductor for the negative grid G is shown at 3, and that for the positive grid G is shown at 4. The plates D D have the outer end spaced from the grid wires 0 by a distance of 1 mm. and the other end from the electrode L by a distance of 0.5 mm. The diameter of the smaller part of the electrode D D is 14 mm. and of the wider part 20 mm. The length 01 the smaller and larger parts is the same, both being 6 mm., all the dimensions being internal dimensions. a

' The electrode L is a disc or molybdenum 0.5 mm. thick, with an aperture of the shape seen in Figure 2, which i 14 mm. across the flats and has a diameter of 16 mm. for the arc-shaped part. The suppressor electrode S is spaced a distance aaeaese of 2.5 mm. from the electrode L, and is a disc similar to L but having an aperture 0.5 mm.

larger all round than the aperture of the electrode L The electrode L has two lead-in wires 9, 9" and the suppressor electrode S a lead-in The slot s is 4 mm. wide and 20 mm. long. The.

sub-anode SA is sealed to the two parts of the glass envelope 'E' by means of fln-like flanges 5. B, and in order to avoid the glass breaking away, the section of these flanges has a pronounced taper towards the root, but the points are thin with practically parallel sides.

It will be observed that all of the electrodes except the cathode C and the deflection cylinder D, D so far described, are each provided with a pair of spaced apertures I, as is also the mica disc 112; these apertures are aligned with two screw-threaded holes 8 in the sub-anode SA for use in assembling and aligning the electrodes.

as is described in my copending application Serial No. 409,589, filed Septembe 4, 1941.

This method preserves perfect alignment which is very important indeed since unless it is preserved, very indifferent results are obtained.

The anode or target T has its front surface spaced 10 mm. from the nearest surface of the sub-anode SA. It is of the shape shown, being sealed in the example illustrated to a re-entrant seal, although in some cases, in order to render it more accessible for cooling, 9. re-entrant seal need not be used. It is united to the seal as shown in Figure 1 by an annular fin H, and it has a screw-threaded socket t in its rear surface for receiving the conductor by which connection is made to it. The anode T is placed behind the slot s in the sub-anode SA.

The beam of electrons is drawn from the cathode C by the positive potential on the accelerating grid G the wires of which are in alignment with and shielded by those of the negative grid G The beam passes through the deflecting and focussing cylinder D D and when there are no deflecting potentials, it passes wholly through the slot 8 in the sub-anode SA and strikes the target T. For the reasons already explained, potentials are such that a focussed beam is produced of rectangular ribbonlike cross-section so as to conform to the slot 5, and is consequently deflected by the electrode D, D in the direction of the smalle dimension of its cross-section. In this way, a greater sensitivity is obtained and a greater current change per unit of deflecting force than would be the case if the beam were deflected in another direction with respect to its cross-section.

A suitable form of testing circuit in which the tube may be tested as a frequency changer or while bombarded during exhausting. is illustrated in Figure 8. The envelope of the tube and the electrodes with their lead-in conductors have been given the same reference characters as in Figures 1 to '7. The battery for supplying heating current to the cathode is shown at BC. Suitable voltages for the electrodes to enable the results described above to be obtained are as follows: The grid Gr is at the same potential as, or a few volts negative to, the cathode C. The

positive grid G is kep't'at- 1000 voltspositive. The mean potential of the deflector and focussing cylinder D D is 300 volts. The electrode H has the same voltage as the anode T, which is 2500 volts. The suppressor electrode S is at cathode potential and the sub-anode SA is kept at 1200 to 1500 volts positive with respect to the cathode.

The output system comprises a concentric conductor output with a central rod l2, to which the high potential is applied screwed into the socket in the rear of the target T. The rod l2 carries the sliding member [3 of an adjustable metal casing l3, It, the member It being fixed to the subanode SA with a sheet of mica 4| interposed since, as mentioned above, the 'sub-anode SA and the member M, which is connected to the target T through the rod l2, are at different potentials. The member I3 is adjustable in the member M for tuning purposes by any suitable adjusting mechanism such as that set forth in my copending application Serial No. 409,588, filed September 4, 1941. I5 is the small loop for taking ofi the output at the increased frequency and is connected to a di-pole transmitting aerial It by a twin-shielded transmission line 53, the electrical length of which is such that it is tuned to the output wave length.

The high frequency oscillations having a wave length of the order of 1 meter to be applied to the parts D D of the deflecting cylinder which serve as input electrodes, may be generated by the circuit shown to the left of Figure 8. Two matched triode tubes T and T of which the filament pins are at the opposite ends to the leadin conductors for the grid and anode, have their cathodes supplied from a battery ll. The cathodes are cross-connected to four Lecher wires p, q. r, s, the slides l8, IQ of which are connected together through a small condenser 20 of 50 micromicrofarads to form the arms of a bridge which also contain two resistances 2!, 22 each of 200 ohms. The junction of these resistances is connected through a milliammeter 23 reading to 20 milliamperes, through resistances 24, 25 of 10,000 ohms each, to the grids of the tubes T T These grids are connected by a condenser k of about 40 micro-microfarads, and the resistance 2d, 25 serve as the grid leaks.

The oscillations generated are taken off from the two anodes by a twin-shielded conductor 26 and applied to the parts D D of the deflecting cylinder. High frequency choke coils 21 are connected in circuit with the cathode battery I1, and further high frequency choke coils 28 are connected in the high tension battery leads to the anodes of the tubes T T The high tension battery HB applies a variable voltage of about 700 between the cathodes and anodes of the tubes T and T and the cathode C of the deflection tube is connected to a tapping t in the battery HB at a point about 300 volts from its positive end. The complete oscillatory input circuit is shielded in a metal box 29. The input driving voltage to the deflection tube may be adjusted by altering the tapping t at the negative end of the 700 volt battery HB so as to vary the high tension voltage of the oscillator. 1

With the dimensions and voltages given above, the following are details of two sets of operating conditions respectively with a watt and a 20 watt output; it should be noted, however, that the following figures represent typical low power output conditions and that very much greater outputs are obtainable.

Drive voltage to deflection plates.. 700 V, peak or the equivalent 50-cycle voltage necessary to produce the same direct current components to the anode and sub-anode. Power output into a, radiating dipole watts (approx) 100 Input wave length -centimetres 100 Output wave length do 50 ZO-watt output Electrode Voltages Currents Modulating grid. Esh=300 Ish== Positive grid Egl=l,25fl Igl=0.l niA. Deflection plate bias, Edo=300 Ido=a few microamps. Accelerating grid :lEE =2,500 fl=1 mA. Su ressor =zeroi Su uode. E;1 =l,500 183212 mA}whcn I Target, Ft=2 500 lt=l mA operating Drive voltage to deflection plates V. peak 700 Power output into a radiating dipole watts (approx.) 20 Input wave length centimetres 100 Output wave length do 50 I claim:

1. An electron discharge device comprising an electron emitting cathode, an anode, an accelerating grid formed of a plurality of parallel grid wires mounted closely in front of said cathode, an electron lens and beam-deflecting system positioned in front of said grid for focusing substantially all of the electrons passing through said grid into a beam and for deflecting said beam in a direction parallel to the direction of said grid wires.

'2. An electron discharge device comprising an electron emitting cathode having an extended electron emitting area of substantially circular shape, an accelerating grid formed of a plurality of parallel grid wires mounted closely in front of said cathode, a conducting plate mounted in front of said grid in spaced relation thereto and having an elongated aperture formed therein at right angles to said parallel grid wires, and an electrode system including an electron lens positioned between said grid and said plate for focusing substantially all of the electrons passing through said grid into a beam directed towards said plate, said beam having an elongated crosssection with its narrow dimension parallel with the narrow dimension of said aperture.

3. An electron discharge device according to claim 2 wherein said electrode system includes means to deflect said beam in a direction parallel with said grid wires.

4. An electron discharge device comprising an electron emitting cathode having an extended electron emitting area, an formed of a plurality of parallel grid wires mounted closely in front of said cathode, an electrode system positioned in front of said grid for focusing substantially all of the electrons passing through said grid intoa beam having an elongated cross-section with its narrow dimension accelerating grid parallel with said grid wires, and including means for deflecting said beam in a direction parallel with said grid wires.

. 5. Anelectron discharge device according to claim 4 wherein said beam deflecting means comprises a pair' of deflecting electrodes which form the two halves of a focusing electrode.

6. An electron discharge device comprising an electron emitting cathode, an accelerating grid formed ofa plurality of parallel grid wires mounted closely in front of said cathode and prod-ucing an electron beam; having a substantially circular cross-section, an electrode system positioned in front of said grid for acting on said beam to expand the cross-section thereof in a direction at right angles to the direction of said grid wires and to contract the cross-section of the beam in a direction parallel with said grid wires,

said electrode system including means for defleeting said beam in a direction parallel with said grid wires.

7. An electron discharge device according to claim 6 and including a shielding grid mounted between said cathode and said accelerating grid and comprising a plurality of parallel grid wires providing an active surface of the same configuration as that of said accelerating grid and located in registry with the parallel grid wires of said accelerating grid.

8. An electron discharge device comprising an electron-emitting cathode, an anode, an accelerating grid comprising a plurality of parallel grid wires mounted closely in front of said cathode, a beam-deflecting system consisting of an electrode located in front of said grid to influence the electron stream emitted from said cathode after passing said grid, and formed as a figure of revolution split into two halves along a plane at right angles to the direction of said grid wires. a ring-shaped accelerating electrode located bea tween said beam-deflecting system and said :in-

ode, and a ring-shaped suppressor electrode mounted between said accelerating electrode and said anode.

9. An electron discharge device comprising a cathode, an anode, an accelerating grid compris ing a plurality of parallel grid wires mounted closely in front of said cathode, a shielding grid mounted between said cathode and said accelerating grid and comprising a plurality of parallel grid wires providing an active surface of the same configuration as that of said accelerating grid and located in registry with the parallel grid wires of said accelerating grid, an electron-lens system in front of said cathode and grids to form an electron beam and including a ring-shaped electrode divided along a plane at right angles to said grid wires, the two parts of said divided electrode comprising deflecting electrodes to produce deflection of the beam in a direction parallel to said grid wires.

'10. An electron discharge device comprising a cathode, an anode, an accelerating grid comprising a plurality of parallel grid wires mounted closely in front of said cathode, an electron-lens system in front of said cathode and grid to form an electron beam and comprising a single pair of deflecting electrodes symmetrically mounted 

